NOx sensor and method of measuring NOx

ABSTRACT

Disclosed is a NOx sensor and a method of measuring NOx capable of obtaining a large change in signal to measure a concentration of low concentration NOx in a measurement gas continuously and accurately with good response over a long period of time. A NOx sensor comprises a first internal space into which the measurement gas is introduced through a first diffusion rate-determining passage, a second internal space arranged with a NOx-reducing catalyst, into which an atmosphere is introduced through a second diffusion rate-determining passage, an electrochemical pumping cell for controlling a partial pressure of oxygen in the internal space by using a first oxygen ion-conductive solid electrolyte and electrochemical cells provided in contact therewith, a partial oxygen pressure-detecting means for detecting the partial pressure of oxygen in the internal space by using the first oxygen ion-conductive solid electrolyte and electro-chemical cells provided in contact therewith, a first electrochemical sensor cell for outputting an electromotive force corresponding to the partial pressure of oxygen in the internal space, and a voltage-detecting means for detecting the electromotive force outputted from the first electrochemical sensor cell. The NOx concentration is determined from a value of the electromotive force of the first electrochemical sensor cell detected by the voltage-detecting means.

This is a Continuation of application Ser. No. 08/732,599 filed Oct. 15,1996, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a NOx sensor and a method of measuringNOx. In particular, the present invention relates to improvement in asensor directed to a combustion gas as a measurement gas, for measuringNOx as a measurement gas component in such a gas, and a method capableof advantageously measuring NOx.

2. Description of the Related Art

Various measuring methods and devices have been proposed for determiningthe concentration of NOx in a measurement gas. A known method, forexample, employs a sensor comprising a Pt electrode and an Rh electrodeformed on an oxygen ion-conductive solid electrolyte such as zirconia.This method utilizes the ability of Rh to reduce NOx so that anelectromotive force generated between the two electrodes is measured.However, a problem arises in that such a sensor tends to suffer from aninfluence of noise, since the electromotive force varies to a greatextent depending on a change in concentration of oxygen contained in acombustion gas as a measurement gas, while the electromotive forcevaries to a small extent in response to a change in concentration ofNOx. On the other hand, a reducing gas such as CO is indispensable forsuch a sensor in order to induce the ability to reduce NOx. However, ingeneral, a large amount of NOx is produced under a combustion conditionconcerning an excessively small amount of fuel, in which an amount ofproduced CO is less than the amount of produced NOx, resulting in adrawback that measurement cannot be performed for a combustion gasdischarged under such a combustion condition.

Another method of measuring NOx is also known, based on a combination ofa set of electrochemical pumping cell and sensing cell including Ptelectrodes and oxygen-ion conductive solid electrolyte, and another setof electrochemical pumping cell and sensing cell including Rh electrodesand oxygen-ion conductive solid electrolyte, as disclosed in JapaneseLaid-open Patent Publication Nos. 63-38154 and 64-39545. In this method,NOx is measured on the basis of a difference between values of pumpingcurrents. Other methods are disclosed, for example, in JapaneseLaid-open Patent Publication Nos. 1-277751 and 2-1543. In one method,two pairs, i.e., a first pair and a second pair of electrochemicalpumping cell and sensing cell are prepared. A limiting pumping currentis measured by using a sensor comprising the first pair of pumping andsensing cells, under a partial pressure of oxygen at which NOx is notreduced, while a limiting pumping current is measured by using a sensorcomprising the second pair of pumping and sensing cells, under a partialpressure of oxygen at which NOx is reduced, so that a difference betweenthe measured limiting pumping currents is determined. In another method,a sensor comprising a pair of pumping cell and sensing cell is used, inwhich a difference in limiting current is measured by switching thepartial pressure of oxygen in a measurement gas between a partialpressure of oxygen at which NOx is reduced and a partial pressure ofoxygen at which NOx is not reduced.

In the aforementioned methods of measuring NOx, however, an extremelysmall part of the value of the limiting current is based on theobjective NOx, and the most part of the value of the limiting current isoccupied by electric power caused by oxygen contained in a large amountin ordinary cases. Therefore, a small current value corresponding to NOxis determined from a difference between two large current values.Accordingly, in the case of the method based on the use of the one setof sensor, problems arise in that the NOx cannot be continuouslymeasured, the operating response is inferior, and the accuracy isinferior. On the other hand, in the case of the method based on the useof the two sets of sensors, an error is likely to occur in a measuredvalue if the oxygen concentration in a measurement gas greatly changes.Therefore, this method cannot be employed in automobile applications,for example, where the oxygen concentration in a measurement gas variesto a large extent. This inconvenience arises from the fact that thedependency of pumping current on oxygen concentration concerning onesensor is different from that concerning the other sensor. In the caseof an automobile, for example, the oxygen concentration in exhaust gasis generally several percentages under a running condition of anair/fuel ratio of 20, whereas the NOx concentration is several hundredsof ppm. The concentration of NOx is about 1/100 of the concentration ofoxygen. Therefore, only a slight difference in the dependency of pumpingcurrent on oxygen concentration brings about a situation in which adifference in the limiting current value corresponding to a change inoxygen concentration is larger than an amount of change in the limitingcurrent based on NOx to be measured. In addition, if a diffusionrate-determining means formed in the pumping cell is clogged with oilash in the exhaust gas, the pumping current may be undesirably changed,resulting in reduced accuracy. Further, if the temperature of theexhaust gas greatly varies, a measured value may involve someabnormality. Moreover, a difference in chronological change in anycharacteristic between the two sensors, if any, may directly lead tomeasuring errors, resulting in a drawback that the entire system is madeundurable for use over a long period of time.

The oxygen present in the measurement gas causes various problems uponmeasurement of NOx, as described above. Accordingly, it has beenstrongly desired to solve these problems.

In order to solve the problems described above, the present inventorshave revealed a new measuring system in Japanese Patent Application No.7-48551. In this system, a measurement gas component having bondedoxygen, such as NOx, contained in a measurement gas can be measuredcontinuously and accurately with good response over a long period oftime without being affected by the oxygen concentration in themeasurement gas or any change thereof, by utilizing first and secondelectrochemical pumping cells arranged in series.

Namely, in the previously proposed new system, a measurement gascontaining a measurement gas component having bonded oxygen to bemeasured is successively introduced from an external measurementgas-existing space into first and second processing zones underpredetermined diffusion resistances respectively. At first, in the firstprocessing zone, the partial pressure of oxygen is controlled to have alow value which does not substantially affect measurement of an amountof the measurement gas component by pumping out oxygen in the atmosphereby using a first electrochemical pumping cell. In the second processingzone, the measurement gas component in the atmosphere introduced fromthe first processing zone is reduced or decomposed. Oxygen produced bythe reduction or decomposition is pumped out by the aid of anoxygen-pumping action effected by a second electrochemical pumping cell.Thus a pumping current flowing through the second electrochemicalpumping cell is detected to obtain a detected value from which theamount of the measurement gas component in the measurement gas isdetermined.

However, as a result of further investigation by the present inventorson such a new measuring system, the sensor concerning the new measuringsystem described above has been clarified to have the following problem.Namely, measurement of a measurement gas component at a lowconcentration, for example, measurement of NOx at several ppm gives apumping current of about several tens of nA detected by the secondelectrochemical pumping cell, which is small as a detection signal.

SUMMARY OF THE INVENTION

Thus the present invention has been made in order to solve the problemsinvolved in the NOx sensor concerning the previously proposed new systemfor measuring the NOx concentration in the measurement gas, an object ofwhich is to provide a NOx sensor and a method of measuring NOx, in whicha large change in signal can be obtained upon measurement of aconcentration of low concentration NOx in a measurement gas, making itpossible to perform measurement continuously and accurately with goodresponse over a long period of time.

In order to achieve the object described above, according to asignificant aspect of the present invention, there is provided a NOxsensor for measuring an amount of NOx in a measurement gas by measuringan amount of oxygen produced by reducing NOx in the measurement gas witha NOx-reducing catalyst, the NOx sensor comprising (a) a first internalspace communicating with an external measurement gas-existing space, (b)a first diffusion rate-determining means for introducing the measurementgas from the measurement gas-existing space into the first internalspace under a predetermined diffusion resistance, (c) an electrochemicalpumping cell comprising a first oxygen ion-conductive solid electrolyteand a pair of electrodes provided in contact therewith, for pumping outoxygen from the first internal space by applying electric power betweenthe pair of electrodes so that a partial pressure of oxygen in anatmosphere in the first internal space is controlled to have apredetermined low value at which NOx is not substantially reduced, (d) asecond internal space communicating with the first internal space andcomprising the NOx-reducing catalyst arranged therein, (e) a seconddiffusion rate-determining means for introducing the controlledatmosphere in the first internal space into the second internal spaceunder a predetermined diffusion resistance, (f) a first electrochemicalsensor cell comprising a second oxygen ion-conductive solid electrolyteand a pair of electrodes provided in contact therewith, for reducing NOxexisting in an atmosphere in the second internal space with theNOx-reducing catalyst, and outputting an electromotive forcecorresponding to a partial pressure of oxygen in the atmosphere in thesecond internal space, defined by oxygen produced by the reduction ofNOx, and (g) a voltage-detecting means for detecting the electromotiveforce outputted from the first electrochemical sensor cell.

Therefore, according to the NOx sensor (first NOx sensor) concerning thepresent invention as described above, the atmosphere in the firstinternal space, which has its partial pressure of oxygen controlled tohave the predetermined low value by the aide of the electrochemicalpumping cell, is introduced into the second internal space communicatingwith the first internal space through the second diffusionrate-determining means. NOx is reduced or decomposed by the NOx-reducingcatalyst in the second internal space. The partial pressure of oxygen inthe second internal space is increased by oxygen produced by thereduction of NOx. Thus a pressure gradient of partial pressure of oxygenis brought about between the second internal space and the firstinternal space. Oxygen pretends to flow in a direction to counteract thepressure gradient, i.e., from the second internal space to the firstinternal space. However, the pressure resistance is maintained by thepredetermined diffusion resistance possessed by the second diffusionrate-determining means. Thus the partial pressure of oxygen in thesecond internal space is equilibrated at a value which is higher thanthe partial pressure of oxygen in the first internal space,corresponding to a concentration of oxygen produced by the reduction ordecomposition of NOx, in other words, corresponding to a NOxconcentration in the measurement gas. The electromotive force, whichcorresponds to the partial pressure of oxygen in the atmosphere in thesecond internal space, is detected on the basis of the output from theelectrochemical sensor cell. Accordingly, even when the amount ofproduced oxygen is minute upon measurement of NOx concentration of a lowconcentration, a large change in electromotive force can be measured.

In a preferred embodiment of the NOx sensor according to the presentinvention, the NOx-reducing catalyst also serves as the electrodearranged in the second internal spaced, of the pair of electrodes forconstituting the electrochemical sensor cell. Accordingly, the NOxsensor can be formed in a compact conformation.

In another preferred embodiment, the NOx sensor according to the presentinvention comprises a sensor element having an integrated structureincluding the first and second oxygen ion-conductive solid electrolytes,wherein the sensor element integrally includes the first and secondinternal spaces, the first and second diffusion rate-determining means,the electrochemical pumping cell, and the electrochemical sensor cell.Accordingly, the NOx sensor according to the present invention can beformed in a more compact conformation. Moreover, the NOx sensoraccording to the present invention can be advantageously produced fromthe viewpoint of cost and applicability to mass production.

In still another preferred embodiment, the NOx sensor according to thepresent invention further comprises a partial oxygen pressure-detectingmeans for detecting the partial pressure of oxygen in the atmosphere inthe first internal space, wherein the partial pressure of oxygen in theatmosphere in the first internal space is controlled by controlling anamount of electric power application between the pair of electrodes ofthe electrochemical pumping cell on the basis of a value of partialpressure of oxygen detected by the partial oxygen pressure-detectingmeans. According to this embodiment, the partial pressure of oxygen inthe first internal space can be maintained at a predetermined value witha high degree of accuracy.

In still another preferred embodiment of the NOx sensor according to thepresent invention, a reference gas-existing space is provided in theintegrated structure of the sensor element independently from the firstand second internal spaces, and the partial oxygen pressure-detectingmeans is constructed by a second electrochemical sensor cell comprisingan oxygen ion-conductive solid electrolyte extending between thereference gas-existing space and the first internal space, a secondreference electrode provided in contact with the solid electrolytelocated on the reference gas-existing space, and a second measuringelectrode provided in contact with the solid electrolyte located on thefirst internal space. According to such an arrangement, an absolutevalue of the partial pressure of oxygen in the first internal space isdetermined easily and accurately by the second electrochemical sensorcell as the partial oxygen pressure-detecting means on the basis of aknown partial pressure of oxygen in the reference gas-existing space.Moreover, the NOx sensor according to the present invention can beformed in a more compact conformation.

In still another preferred embodiment of the NOx sensor according to thepresent invention, the reference gas-existing space is open at anaperture of the sensor element exposed to atmospheric air, andatmospheric air as a reference gas is introduced into the referencegas-existing space through the aperture. Accordingly, it is realized,based on a simple structure, that the absolute value of the partialpressure of oxygen in the first internal space is detected on the basisof a partial pressure of oxygen of atmospheric air by the aid of thesecond electrochemical sensor cell as the partial oxygenpressure-detecting means.

In still another preferred embodiment of the NOx sensor according to thepresent invention, the first electrochemical sensor cell uses the oxygenion-conductive solid electrolyte extending between the second internalspace and the reference gas-existing space, as the second oxygenion-conductive solid electrolyte, and the pair of electrodes of thefirst electrochemical sensor cell are constructed by a first measuringelectrode provided in contact with the solid electrolyte located on thesecond internal space and a first reference electrode provided incontact with the solid electrolyte located on the reference gas-existingspace. According to this embodiment, an absolute value of the partialpressure of oxygen in the second internal space is determined easily andaccurately by the first electrochemical sensor cell on the basis of aknown partial pressure of oxygen in the reference gas-existing space.Moreover, the NOx sensor according to the present invention can beformed in a more compact conformation.

In still another preferred embodiment of the NOx sensor according to thepresent invention, a layer of the NOx-reducing catalyst is provided onthe first measuring electrode. Accordingly, it is possible to use, forthe first measuring electrode, a material which is poor in ability toreduce NOx at a temperature at which the NOx sensor is used, therebyincreasing the degree of freedom of material selection for the firstmeasuring electrode.

In still another preferred embodiment of the NOx sensor according to thepresent invention, the second diffusion rate-determining means and/orthe second internal space is composed of a porous material having apredetermined diffusion resistance. When this embodiment is adopted, thesensitivity for detecting the NOx concentration can be improved owing tothe diffusion resistance possessed by the porous material. The degree offreedom of design is increased, for example, for the shape andarrangement position of the second diffusion rate-limiting means and/orthe second internal space. Thus both of the improvement in performanceof the NOx sensor and the applicability to mass production can beachieved.

In still another preferred embodiment of the NOx sensor according to thepresent invention, the second diffusion rate-determining means and thesecond internal space are constructed by one porous material having apredetermined diffusion resistance, and arranged in the first internalspace, and the first measuring electrode and the second measuringelectrode are arranged adjacent to one another with respect to adirection of diffusion of the atmosphere in the first internal space. Itis desirable that the first measuring electrode and the second measuringelectrode are arranged as close as possible with respect to thediffusing direction.

According to the NOx sensor of the present invention having thearrangement as described above, the second diffusion rate-determiningmeans and the second internal space are constructed by one porousmaterial having the predetermined diffusion resistance, and arranged inthe first internal space. Accordingly, the structure is simplified, andthe applicability to mass production is further improved. Moreover, thefirst measuring electrode and the second measuring electrode arearranged adjacent to one another with respect to the direction ofdiffusion of the atmosphere in the first internal space. Accordingly, itis possible to decrease the change in electromotive force of the firstelectrochemical sensor cell, i.e., the measuring error for the NOxconcentration, caused by the change in distribution of oxygenconcentration existing in the gas-diffusing direction depending on theoxygen concentration in the measurement gas.

In still another preferred embodiment of the NOx sensor according to thepresent invention, the first measuring electrode and the secondmeasuring electrode are arranged in parallel with respect to thedirection of diffusion of the atmosphere in the first internal space.Accordingly, it is possible to further increase the effect to decreasethe measuring error for the NOx concentration, caused by the change indistribution of oxygen concentration existing in the gas-diffusingdirection depending on the oxygen concentration in the measurement gas.

In still another preferred embodiment, the NOx sensor according to thepresent invention further comprises a third electrochemical sensor cellfor detecting, in the second internal space, the partial pressure ofoxygen in the atmosphere introduced from the first internal space intothe second internal space through the second diffusion rate-determiningmeans, wherein control of the electrochemical pumping cell, which iseffected by the second electrochemical sensor cell, is corrected on thebasis of a value of partial pressure of oxygen detected by the thirdelectrochemical sensor cell.

According to the NOx sensor of the present invention having thearrangement as described above, the control of the electrochemicalpumping cell, which is effected by the second electrochemical sensorcell, is corrected on the basis of the value of partial pressure ofoxygen in the atmosphere in the vicinity of the first electrochemicalsensor cell in the second internal space. Accordingly, the value ofpartial pressure of oxygen is stably maintained to be constant. Thus itis possible to decrease the change in electromotive force of the firstelectrochemical sensor cell, i.e., the measuring error for the NOxconcentration, even if the change or the error is extremely large,caused by the change in distribution of oxygen concentration existing inthe measurement gas depending on the oxygen concentration in themeasurement gas.

In still another preferred embodiment of the NOx sensor according to thepresent invention, a porous material layer having a predetermineddiffusion resistance is provided around the first measuring electrode ofthe first electrochemical sensor cell arranged in the second internalspace. Accordingly, the partial pressure of oxygen in the porousmaterial layer around the first measuring electrode is equilibrated,owing to the diffusion resistance possessed by the porous materiallayer, at a value which is higher than the partial pressure of oxygen inthe second internal space, corresponding to the NOx concentration in themeasurement gas. Thus the detecting sensitivity of the NOx sensor isincreased.

In still another preferred embodiment, the NOx sensor according to thepresent invention further comprises a heating means capable of heatingthe first internal space and the second internal space to apredetermined temperature respectively. Accordingly, it is possible toincrease the accuracy in detection of partial pressure of oxygeneffected by each of the electrochemical sensor cells. Moreover, anadvantage is obtained in that the electrochemical pumping cell can besubjected to the pumping action more effectively even when thetemperature of the measurement gas is low, or it changes.

According to another significant aspect of the present invention, thereis provided another NOx sensor (second NOx sensor) capable of achievingthe object described above, which lies in a NOx sensor, based on the useof a sensor element having an integrated structure comprising oxygenion-conductive solid electrolytes, for measuring an amount of NOx in ameasurement gas by measuring an amount of oxygen produced by reducingNOx in the measurement gas with a NOx-reducing catalyst arranged in aninternal space provided in the sensor element, the NOx sensor comprising(h) a first diffusion rate-determining means for introducing themeasurement gas from an external measurement gas-existing space into theinternal space in the sensor element under a predetermined diffusionresistance, (i) an electrochemical pumping cell comprising an oxygenion-conductive solid electrolyte of the sensor element and a pair ofelectrodes provided in contact therewith, for pumping out oxygen fromthe internal space by applying electric power between the pair ofelectrodes so that a partial pressure of oxygen in an atmosphere in theinternal space is controlled to have a predetermined low value at whichNOx is not substantially reduced, (j) a first electrochemical sensorcell comprising an oxygen ion-conductive solid electrolyte of the sensorelement, and a first measuring electrode and a first reference electrodeprovided in contact therewith, the first measuring electrode beinglocated on the internal space and having a porous structure to functionas the NOx-reducing catalyst as well, and the first reference electrodebeing located on a reference gas-existing space provided in the sensorelement, wherein NOx existing in an atmosphere in the internal space isreduced in the porous structure of the first measuring electrode, and anelectromotive force is outputted corresponding to a partial pressure ofoxygen in an atmosphere in the porous structure, defined by oxygenproduced by the reduction of NOx, (k) a constant current power source,provided on an electromotive force-outputting circuit including thefirst measuring electrode and the first reference electrode of the firstelectrochemical sensor cell, for allowing a constant current to flow sothat oxygen is pumped out from the first measuring electrode to thefirst reference electrode, and allowing the partial pressure of oxygenin the atmosphere in the porous structure of the first measuringelectrode to have a predetermined value at which NOx is reduced, and (l)a voltage-detecting means, provided on the electromotiveforce-outputting circuit of the first electrochemical sensor cell, fordetecting the electromotive force outputted from the firstelectrochemical sensor cell.

According to the second NOx sensor of the present invention as describedabove, the oxygen exists in the atmosphere in the internal space havingthe partial pressure of oxygen controlled to have the predetermined lowvalue by the aid of the electrochemical pumping cell. The oxygen isintroduced into the inside of the first measuring electrode in an amountalways corresponding to a previously set current value of the constantcurrent power source in a state in which the partial pressure of oxygenin the porous structure is lower than that in the internal space by anamount corresponding to pressure loss involved in the diffusionresistance possessed by the first measuring electrode having the porousstructure, by the aid of the pumping action effected by the constantcurrent power source. Subsequently, the oxygen is pumped out from theinside of the first measuring electrode to the reference gas-existingspace. On the other hand, NOx in the atmosphere diffuses into the firstmeasuring electrode under the predetermined diffusion resistance, whichis reduced or decomposed by the first measuring electrode in thevicinity of the surface of the first measuring electrode. The partialpressure of oxygen in the atmosphere in the porous structure of thefirst measuring electrode is increased by oxygen produced by thereduction or decomposition of NOx. However, the pressure of oxygen inthe porous structure is lower than that in the internal space by anamount corresponding to pressure loss when the NOx concentration iszero. In addition, the first measuring electrode has the diffusionresistance. Accordingly, the produced oxygen in the first measuringelectrode scarcely diffuses to the internal space. Therefore, any changein NOx concentration in the measurement gas results in a large change inpartial pressure of oxygen in the porous structure. Thus the constantcurrent power source greatly changes the voltage applied to the firstelectrochemical sensor cell in order to continuously allow the currenthaving the previously set value to flow through the electromotiveforce-outputting circuit, depending on the change in partial pressure ofoxygen in the porous structure of the first measuring electrode. Thevoltage-detecting means detects the change in voltage applied to thefirst electrochemical sensor cell by the constant current power source,corresponding to the partial pressure of oxygen in the atmosphere in theporous structure, defined by the produced oxygen as described above.Accordingly, even a slight amount of produced oxygen can be measured asa large change in voltage. Thus it is possible to increase thesensitivity for detecting the NOx concentration.

In a preferred embodiment, the second NOx sensor according to thepresent invention further comprises a partial oxygen pressure-detectingmeans for detecting the partial pressure of oxygen in the atmosphere inthe internal space, wherein the partial pressure of oxygen in theatmosphere in the internal space is controlled by controlling an amountof electric power application between the pair of electrodes of theelectrochemical pumping cell on the basis of a value of partial pressureof oxygen detected by the partial oxygen pressure-detecting means.Accordingly, the partial pressure of oxygen in the atmosphere in theinternal space can be maintained at a predetermined value with a highdegree of accuracy, in the same manner as the first NOx sensor describedabove.

In another preferred embodiment of the second NOx sensor according tothe present invention, the partial oxygen pressure-detecting means isconstructed by the oxygen ion-conductive solid electrolyte of the sensorelement, a second measuring electrode located on the internal space,provided in contact therewith, and a second reference electrode locatedon the reference gas-existing space. Accordingly, the NOx sensoraccording to the present invention can be formed in a more compactconformation in the same manner as the first NOx sensor described above.

In still another preferred embodiment of the second NOx sensor accordingto the present invention, a porous material layer having a predetermineddiffusion resistance is provided around the first measuring electrode ofthe electrochemical sensor cell. Accordingly, it is possible to moreeffectively suppress the diffusion of oxygen produced by the reductionor decomposition of NOx, from the first measuring electrode to theinternal space. Thus the sensitivity for detecting the NOx concentrationis further improved.

In still another preferred embodiment of the second NOx sensor accordingto the present invention, the first and second measuring electrodes arearranged adjacent to one another with respect to a direction ofdiffusion of the atmosphere in the internal space. Accordingly, it ispossible to obtain the function and effect equivalent to those obtainedin the first NOx sensor described above.

In still another preferred embodiment of the second NOx sensor accordingto the present invention, the first and second measuring electrodes arearranged in parallel with respect to the direction of diffusion of theatmosphere in the internal space. Accordingly, it is possible to furtherenhance the effect to decrease the measuring error for the NOxconcentration, caused by the distribution of oxygen concentrationexisting in the gas-diffusing direction, in the same manner as the firstNOx sensor described above.

In still another preferred embodiment of the second NOx sensor accordingto the present invention, the internal space is divided into a firstinternal space communicating with the external measurement gas-existingspace through the first diffusion rate-determining means, and a secondinternal space into which an atmosphere in the first internal space isintroduced through a second diffusion rate-determining means under apredetermined diffusion resistance, the electrochemical pumping cell islocated on the first internal space, the first electrochemical sensorcell is located on the second internal space, and the NOx sensor furthercomprises a subsidiary oxygen-pumping means for lowering a partialpressure of oxygen in the atmosphere introduced from the first internalspace into the second internal space, to a degree sufficient to reduceNOx in the porous structure of the first measuring electrode of thefirst electrochemical sensor cell. Advantageously, the subsidiaryoxygen-pumping means is constructed by the oxygen ion-conductive solidelectrolytes of the sensor element and a pair of electrodes provided incontact therewith.

According to the second NOx sensor of the present invention as describedabove, NOx is successively introduced into the first internal space andthe second internal space. In the respective internal spaces, theoxygen-pumping action is effected by the electrochemical pumping celland the subsidiary oxygen-pumping means to pump out oxygen. Accordingly,in the first internal space, nothing is required other than the oxygenconcentration is lowered to the degree sufficient to control the partialpressure of oxygen by the aid of the subsidiary oxygen-pumping means inthe subsequent second internal space. In the second internal space, thevalue of partial pressure of oxygen, which has been lowered in the firstinternal space, is further lowered so that the partial pressure ofoxygen may be accurately controlled to have the value of partialpressure of oxygen which does not substantially affect measurement ofthe amount of the measurement gas component. Therefore, even when theoxygen concentration of the measurement gas is high, and hence thechange in electromotive force of the first electrochemical sensor cell,caused by the change in distribution of oxygen concentration in themeasurement gas depending on the oxygen concentration in the measurementgas, i.e., the measuring error for measurement of NOx concentration isextremely large, the change in electromotive force generated in thefirst electrochemical sensor cell is not affected by any influencethereof. Thus an obtained value accurately corresponds to the amount ofthe measurement gas component existing in the measurement gas, making itpossible to perform accurate measurement. In addition, the NOxconcentration in the measurement gas can be accurately measured in astate in which the partial pressure of oxygen in the first internalspace is previously set to be a predetermined value which issufficiently higher than the partial pressure of oxygen in the secondinternal space. Accordingly, even when inflammable gases such as CO, HC,and H₂ are present in the measurement gas in a mixed manner, theinflammable gases can be removed by oxidation in the first internalspace. Thus it is possible to decrease the measuring error for the NOxconcentration, due to interference by the inflammable gases in thesecond internal space, as small as possible.

In still another preferred embodiment, the second NOx sensor accordingto the present invention further comprises a heating means capable ofheating the internal space to a predetermined temperature. Accordingly,it is possible to obtain the same function and effect as those obtainedin the first NOx sensor described above.

According to other significant aspects of the present invention, thefollowing methods of measuring a NOx component in a measurement gas arealso provided, wherein the measuring methods may be advantageouslycarried out by using the foregoing NOx sensors.

One of the methods of measuring NOx components in measurement gasesaccording to the present invention is carried out concerning theforegoing first NOx sensor, which lies in a method of measuring NOx,comprising the steps of introducing a measurement gas from an externalmeasurement gas-existing space into a first internal space under apredetermined diffusion resistance, controlling a partial pressure ofoxygen in an atmosphere in the first internal space to have apredetermined low value at which NOx is not substantially reduced, bythe aid of an oxygen-pumping action effected on the first internal spaceby an electrochemical pumping cell, introducing the controlledatmosphere in the first internal space into a second internal spaceunder a predetermined diffusion resistance, reducing, in the secondinternal space, NOx existing in an atmosphere with a NOx-reducingcatalyst, and outputting, with the use of an electrochemical sensorcell, an electromotive force corresponding to a partial pressure ofoxygen in the atmosphere in the second internal space, defined by oxygenproduced by the reducing step to obtain a detected output value fromwhich an amount of NOx in the measurement gas is determined.

Preferably, in the measuring method according to the present invention,the partial pressure of oxygen in the atmosphere in the first internalspace is controlled to have the constant value by detecting the partialpressure of oxygen in the atmosphere in the first internal space toobtain a detected value on the basis of which a power source voltage ischanged so that the oxygen-pumping action effected by theelectrochemical pumping cell is controlled.

According to the present invention, there is provided another measuringmethod concerning the foregoing second NOx sensor, which lies in amethod of measuring NOx, comprising the steps of introducing ameasurement gas from an external measurement gas-existing space into aninternal space under a predetermined diffusion resistance, controlling apartial pressure of oxygen in an atmosphere in the internal space tohave a predetermined low value at which NOx is not substantiallyreduced, by the aid of an oxygen-pumping action effected on the internalspace by an electrochemical pumping cell, reducing NOx in a porousstructure by allowing a constant current to flow through anelectrochemical sensor cell comprising a measuring electrode having theporous structure located on the internal space so that a partialpressure of oxygen in an atmosphere in the porous structure has apredetermined value at which NOx is reduced, and outputting, with theuse of the electrochemical sensor cell, an electromotive forcecorresponding to a partial pressure of oxygen defined by oxygen producedby the reducing step to obtain a detected output value from which anamount of NOx in the measurement gas is determined.

Preferably, in the second measuring method according to the presentinvention, the partial pressure of oxygen in the atmosphere in theinternal space is controlled to have the constant value by detecting thepartial pressure of oxygen in the atmosphere in the internal space toobtain a detected value on the basis of which a power source voltage ischanged so that the oxygen-pumping action effected by theelectrochemical pumping cell is controlled.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

DESCRIPTION OF THE DRAWINGS

FIG. 1(A) explanatorily shows a plan view of a first embodiment of theNOx sensor according to the present invention, and FIG. 1 (B)explanatorily shows an enlarged view of principal components, takenalong a cross section of I--I shown in FIG. 1 (A).

FIG. 2 (A) explanatorily shows a plan view corresponding to FIG. 1 (A)of a modified embodiment of the NOx sensor according to the presentinvention, and FIG. 2 (B) explanatorily shows a cross-sectional viewcorresponding to FIG. 1 (B).

FIG. 3 (A) explanatorily shows a plan view corresponding to FIG. 1 (A)of another modified embodiment of the NOx sensor according to thepresent invention, and FIG. 3 (B) explanatorily shows an enlarged viewof principal components, taken along a cross section of II--II shown inFIG. 3 (A).

FIG. 4 (A) explanatorily shows a cross-sectional view corresponding toFIG. 1 (B) of still another modified embodiment of the NOx sensoraccording to the present invention, and FIG. 4 (B) shows a block diagramillustrating a system of pumping voltage control effected in the NOxsensor shown in FIG. 4 (A).

FIG. 5 (A) explanatorily shows a plan view corresponding to FIG. 1 (A)of a second embodiment of the NOx sensor according to the presentinvention, and FIG. 5 (B) explanatorily shows a cross-sectional viewcorresponding to FIG. 1 (B).

FIG. 6 explanatorily shows a modified embodiment of the secondembodiment of the NOx sensor according to the present invention,explanatorily illustrating a cross-sectional view corresponding to FIG.1 (B).

FIG. 7 shows a graph exemplarily illustrating relationships between theNO concentration in a measurement gas and the electromotive force,obtained in the first and second embodiments of the NOx sensor accordingto the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to clarify the present invention more specifically, the systemof the present invention will be explained in detail below withreference to embodiments illustrated in the drawings.

At first, FIG. 1 (A) and FIG. 1 (B) illustrate a representative exampleof a NOx sensor (first NOx sensor) as a first embodiment according tothe present invention. FIG. 1 (A) shows a plan view of the sensor, andFIG. 1 (B) explanatorily shows an enlarged view of principal components,taken along a cross section of I--I shown in FIG. 1 (A).

With reference to FIG. 1 (A) and FIG. 1 (B), reference numeral 2indicates a sensor element having a slender and lengthy plate-shapedconfiguration. As shown in FIG. 1 (B), the sensor element 2 is aplate-shaped object having an integrated structure comprising aplurality of dense and airtight oxygen ion-conductive solid electrolytelayers 4a, 4b, 4c, 4d, 4e, 4f stacked up with each other. Each of thesolid electrolyte layers 4a to 4f is formed of a known oxygenion-conductive solid electrolyte material such as zirconia ceramics. Thesensor element 2 having the integrated structure is produced by firingstacked unfired solid electrolyte layers into the integrated structurein the same manner as those hitherto performed.

The sensor element 2 having the integrated structure described aboveincludes a first internal space 6 and a second internal space 8 eachhaving a rectangular and plane configuration and individuallyconstructed such that the first internal space 6 is located on a distalside of the element. A reference air-introducing passage 10, whichserves as a reference gas-existing space, extends in a longitudinaldirection of the sensor element 2. The reference air-introducing passage10 is provided independently from the first and second internal spaces6, 8, while overlapping vertically with the first and second internalspaces 6, 8. The reference air-introducing passage 10 is open at theproximal end of the sensor element 2 to make communication withatmospheric air. In this embodiment, the first and second internalspaces 6, 8 are formed such that spaces corresponding thereto formedthrough the solid electrolyte layer 4b are closed by the upper and lowersolid electrolyte layers 4a, 4c. Accordingly, the first and secondinternal spaces 6, 8 are substantially located on an identical plane.The reference air-introducing passage 10 is formed such that a spacecorresponding thereto formed through the solid electrolyte layer 4d isclosed by the upper and lower solid electrolyte layers 4c, 4e.

A first diffusion rate-determining passage 12, which serves as a firstdiffusion rate-determining means for allowing the first internal space 6to communicate with an external measurement gas-existing space, isformed by cutting out the solid electrolyte layer 4b so that the firstdiffusion rate-determining passage 12 is open at the distal end of thesensor element 2. A measurement gas containing NOx is introduced intothe first internal space 6 under a predetermined diffusion resistancethrough the first diffusion rate-determining passage 12. A seconddiffusion rate-determining passage 14, which serves as a seconddiffusion rate-determining means for making communication between thetwo internal spaces 6,8, is formed through the solid electrolyte layer4b located between the first and second internal spaces 6, 8. Anatmosphere in the first internal space 6 is introduced into the secondinternal space 8 under a predetermined diffusion resistance through thesecond diffusion rate-determining passage 14.

An inner pumping electrode 16, which is composed of a rectangular porouscermet electrode, is provided on a portion of the solid electrolytelayer 4a exposed to the first internal space 6, in contact therewith. Anouter pumping electrode 18, which is also composed of a rectangularporous cermet electrode, is provided on an outer surface portion of thesolid electrolyte layer 4a corresponding to the inner pumping electrode16, in contact therewith. An electrochemical pumping cell is constructedby the electrodes 16, 18 and the solid electrolyte layer 4a. A desiredvoltage is applied between the two electrodes 16, 18 of theelectrochemical pumping cell by using an external variable power source20, and a current is allowed to flow in a direction from the outerpumping electrode 18 to the inner pumping electrode 16. Thus oxygen inthe atmosphere in the first internal space 6 can be pumped out to theexternal measurement gas-existing space. The porous cermet electrodecomprises a metal such as Pt and a ceramic such as ZrO₂. However, theinner pumping electrode 16 is arranged in the first internal space 6,which contacts with the measurement gas. Therefore, it is necessary forthe inner pumping electrode 16 to use a metal having a weak ability orno ability to reduce the NOx component in the measurement gas.Desirably, the inner pumping electrode 16 comprises, for example, acermet of ZrO₂ and a Pt-Au alloy. As described above, the pumpingelectrodes 16, 18 generally have a porous structure.

A second measuring electrode 22, which is composed of a porous cermetelectrode similarly to the inner pumping electrode 16, is provided on aportion of the solid electrolyte layer 4c exposed to the first internalspace 6, in contact therewith. A second reference electrode 24, which iscomposed of a porous cermet electrode similarly to the outer pumpingelectrode 18, is provided on a portion of the solid electrolyte layer 4cexposed to the reference air-introducing passage 10, in contacttherewith. An electrochemical cell, which serves as a partial oxygenpressure-detecting means, i.e., a second electrochemical sensor cell, isconstructed by the second measuring electrode 22, the second referenceelectrode 24, and the solid electrolyte layer 4c. The partial pressureof oxygen in the atmosphere in the first internal space 6 is detected bymeasuring an electromotive force generated between the second measuringelectrode 22 and the second reference electrode 24, by using a secondpotentiometer 26, on the basis of a difference in oxygen concentrationbetween the atmosphere in the first internal space 6 and the referenceair (atmospheric air) in the reference air-introducing passage 10, aswell known in the art. The voltage of the variable power source 20 iscontrolled on the basis of a value of partial pressure of oxygen in theatmosphere in the first internal space 6, detected by the secondpotentiometer 26. Thus the pumping action of the electrochemical pumpingcell is controlled so that the partial pressure of oxygen in theatmosphere in the first internal space 6 comes to a predetermined lowvalue at which NOx is not substantially reduced in the presence of theinner pumping electrode 16 and the second measuring electrode 22 of thesecond electrochemical sensor cell.

A rectangular first measuring electrode 28 is provided in the secondinternal space 8, on a portion of the solid electrolyte layer 4c exposedto the second internal space 8, in contact therewith. The firstmeasuring electrode 28 is composed of a porous cermet comprisingzirconia as a ceramic and Rh which is a metal capable of reducing NOx.Thus the first measuring electrode 28 functions as a NOx-reducingcatalyst capable of reducing NOx existing in an atmosphere in the secondinternal space 8. On the other hand, a first reference electrode 30 isprovided on the solid electrolyte layer 4c, corresponding to the firstmeasuring electrode 28, so that the first reference electrode 30 isexposed to the inside of the reference air-introducing passage 10. Afirst electrochemical sensor cell is constructed by the first measuringelectrode 28, the first reference electrode 30, and the solidelectrolyte layer 4c. As well known, an electromotive force, which isgenerated between the first measuring electrode 28 and the firstreference electrode 30, is outputted on the basis of a difference inoxygen concentration between an atmosphere around the first measuringelectrode 28 and an atmosphere around the first reference electrode 30.The electromotive force is measured by a first potentiometer 32. Thus itis possible to detect the partial pressure of oxygen in the atmospherearound the first measuring electrode 28, in other words, the partialpressure of oxygen defined by oxygen produced by reducing or decomposingthe measurement gas component (NOx).

A heater 34, which is heated by external power supply, is embedded inthe sensor element 2 such that the heater 34 is vertically interposedbetween the solid electrolyte layers 4e, 4f. Upper and lower surface ofthe heater 34 are covered with thin layers of ceramic such as alumina,although not shown, in order to obtain electric insulation from thesolid electrolyte layers 4e, 4f. In this embodiment, as shown in FIG. 1(B), the heater 34 is arranged over the entire length ranging from thefirst internal space 6 to the second internal space 8. Thus the internalspaces 6, 8 are heated to a predetermined temperature respectively.Consequently, the electrochemical pumping cell, as a matter of course,as well as the first and second electrochemical sensor cells is heatedto and maintained at the predetermined temperature respectively.

In conformity with the arrangement of the sensor element 2 as describedabove, its distal end is arranged in the measurement gas-existing space.Accordingly, the measurement gas is introduced into the first internalspace 6 under the predetermined diffusion resistance through the firstdiffusion rate-determining passage 12 provided in the sensor element 2.The measurement gas introduced into the first internal space 6 undergoesthe oxygen-pumping action evoked by applying a predetermined voltagebetween the two pumping electrodes 16, 18 which constitute theelectrochemical pumping cell so that the partial pressure of oxygen iscontrolled to have a predetermined value, for example, 10⁻¹⁰ atm.

In order to control the partial pressure of oxygen in the atmosphere inthe first internal space 6 to have the predetermined value, a techniqueis adopted on the basis of the well known Nernst equation. Namely, theelectromotive force between the second measuring electrode 22 and thesecond reference electrode 24 of the electrochemical sensor cell ismeasured by using the second potentiometer 26. The voltage (variablepower source 20) applied between the two electrodes 16, 18 of theelectrochemical pumping cell is controlled so that the electromotiveforce is, for example, 430 mV (700° C.). Thus the partial pressure ofoxygen is controlled to have the objective value of 10⁻¹⁰ atm. Namely,the voltage of the first electrochemical pumping cell is controlled sothat the electromotive force corresponds to a difference between adesired oxygen concentration in the first internal space 6 and an oxygenconcentration in the reference air. The first diffusion rate-determiningpassage 12 serves to reduce the amount of oxygen in the measurement gasdiffusing and flowing into the measuring space (first internal space 6)when the voltage is applied to the first electrochemical pumping cell sothat the current flowing through the electrochemical pumping cell issuppressed.

A state of partial pressure of oxygen, in which NOx in the atmosphere isnot reduced by the inner pumping electrode 16 and the second measuringelectrode 22, for example, a state of partial pressure of oxygen, inwhich the reaction: NO→1/2N₂ +1/2O₂ does not take place, is establishedin the first internal space 6 even under a heated condition caused byheating by the external measurement gas and the heater 34. If NOx in themeasurement gas (or in the atmosphere) is reduced in the first internalspace 6, it is impossible to accurately measure NOx in the secondinternal space 8. In this context, it is necessary to establish thestate in the first internal space 6 in which NOx is not reduced by anycomponent (any metal component of the inner pumping electrode 16 and thesecond measuring electrode 22 in this embodiment) which may concernreduction of NOx.

The measurement gas, which has its partial pressure of oxygen controlledin the first internal space 6 as described above, is introduced into thesecond internal space 8 through the second diffusion rate-determiningpassage 14 under the predetermined diffusion resistance. NOx in themeasurement gas introduced into the second internal space 8 is reducedor decomposed in accordance with, for example, the reaction: NO→1/2N₂+1/2O₂ around the first measuring electrode 28 which also functions as acatalyst for reducing NOx, under the heated condition and under thepartial pressure of oxygen. The first potentiometer 32 measures anelectromotive force generated between the first measuring electrode 28and the first reference electrode 30 on the basis of a differencebetween the partial pressure of oxygen in the atmosphere around thefirst reference electrode 30 and a partial pressure of oxygen in theatmosphere around the first measuring electrode 28, i.e., in theatmosphere in the second internal space 8, defined by oxygen produced bythe reduction or decomposition. Accordingly, it is possible to detectthe partial pressure of oxygen in the atmosphere in the second internalspace 8, in other words, the raised partial pressure of oxygen, definedby oxygen produced by the reduction or decomposition of NOx. Thus theNOx concentration in the measurement gas can be measured. In order toincrease the ratio of change in partial pressure of oxygen in theatmosphere in the second internal space 8 based on oxygen produced asdescribed above, it is desirable that the partial pressure of oxygen inthe atmosphere in the first internal space 6, and consequently thepartial pressure of oxygen in the atmosphere in the second internalspace 8 in the absence of NOx is maintained at a sufficiently low valuewhich is, for example, 10⁻¹⁰ atm.

The partial pressure of oxygen in the atmosphere in the second internalspace 8, which is defined and raised by oxygen produced by reducing ordecomposing NOx in the measurement gas, tends to decrease on account ofdiffusion of oxygen in a direction to counteract the pressure gradientof partial pressure of oxygen between the second internal space 8 andthe first internal space 6, i.e., from the second internal space 8 tothe first internal space 6. However, in the NOx sensor of this firstembodiment, the pressure gradient is not completely counteracted becauseof the predetermined diffusion resistance possessed by the seconddiffusion rate-determining passage 14. The partial pressure of oxygen inthe atmosphere in the second internal space 8 is equilibrated at a valuewhich is higher than the partial pressure of oxygen in the firstinternal space 6, substantially corresponding to the increased partialpressure of oxygen brought about by oxygen produced by reducing ordecomposing NOx in the atmosphere in the second internal space 8, i.e.,substantially corresponding to the NOx concentration in the measurementgas. Thus the electromotive force, which corresponds to the partialpressure of oxygen in the atmosphere in the second internal space 8, isdetected from the output from the electrochemical sensor cell.Accordingly, even when the amount of produced oxygen is minute uponmeasurement of the NOx concentration at a low concentration, the NOxconcentration can be measured as a large change in electromotive force.

The sensitivity for detecting the NOx concentration can be regulated bychanging the cross-sectional area, the cross-sectional shape, and thelength of the second diffusion rate-determining passage 14.

The sensitivity for detecting the NOx concentration can be furtherincreased by packing a porous material having a predetermined diffusionresistance into the second diffusion rate-determining passage 14 and/orthe second internal passage 8, because of the reason as described above.

In the foregoing embodiment, any one of the inner pumping electrode 16and the second measuring electrode 22, arranged in the first internalspace 6, is required not to reduce or decompose NOx in the atmosphererespectively at the ambient temperature and the controlled partialpressure of oxygen in the internal space respectively. Accordingly,those usable include electrode metals such as Au and Ni having noability or a weak ability to reduce or decompose NOx. Thoseadvantageously usable include, for example, cermet electrodes comprisingthe metal described above, and cermet electrodes obtained by using analloy prepared by adding a metal having no catalytic ability such as Auand Ni described above to a noble metal such as Pt, Pd, and Rh.Desirably, the first measuring electrode 28 arranged in the secondinternal space 8 is a cermet electrode composed of, for example, Rhcapable of reducing or decomposing NOx in the atmosphere at theenvironmental temperature and the partial pressure of oxygen in thesecond internal space 8. It is of course possible to use, as the firstmeasuring electrode 28, those obtained by arranging and stacking, on anordinary electrode, an Rh or Pt electrode, or a catalyst materialcomprising a NOx-reducing metal carried on a ceramic porous materialsuch as alumina, and those obtained by arranging an Rh catalystelectrode on a Pt electrode.

In any case, the respective electrodes provided for the NOx sensordescribed above, especially the inner pumping electrode 16 and themeasuring electrodes 22, 28 arranged in the respective internal spacesare desirably cermet electrodes composed of an electrode metal and anappropriate ceramic. In particular, as exemplified above, in the case ofthe use of the first measuring electrode 28 which also functions as aNOx-reducing catalyst, it is desirable to use a porous cermet electrodecomprising a ceramic and a known metal capable of reducing NOx such asRh and Pt. The NOx-reducing catalyst may be provided in the closevicinity of the first measuring electrode 28 of the firstelectrochemical pumping cell for detecting the partial pressure ofoxygen in the second internal space 8. Alternatively, a porous alumina,on which a NOx-reducing catalyst comprising, for example, Rh is carried,may be stacked on the first measuring electrode 28 by means of printingor the like to form a NOx-reducing catalyst layer on the electrode.

It is needless to say that the NOx sensor according to the presentinvention should not be interpreted such that the NOx sensor is limitedto only the structure of the foregoing embodiment. One modifiedembodiment of the NOx sensor is shown in FIG. 2 (A), (B).

The first modified embodiment shown in FIG. 2 (A), (B) is different fromthe foregoing first embodiment, which is specifically characterized inthat the second internal space and the second diffusion rate-determiningpassage as the second diffusion rate-determining means are constructedby one porous material layer 40 having a predetermined diffusionresistance, arranged to cover a first measuring electrode 28, and thefirst measuring electrode 28 and a second measuring electrode 22 arearranged in series adjacent to one another with respect to a directionof diffusion of an atmosphere in an internal space 42. Namely, theporous material layer 40 is constructed to serve as both of the seconddiffusion rate-determining passage and the second internal space. Themeasurement gas, which has its partial pressure of oxygen controlled inthe internal space 42, diffuses through the porous material layer 40under a predetermined diffusion resistance, and arrives at the firstmeasuring electrode 28 arranged at a section under the porous materiallayer 40. Thus NOx in the measurement gas is reduced by the firstmeasuring electrode 28. An electromotive force, which is defined byoxygen produced by the reduction of NOx, corresponding to a raisedpartial pressure of oxygen, is generated between the first measuringelectrode 28 and the first reference electrode 30. The electromotiveforce is measured by a first potentiometer 32. Accordingly, thestructure of the NOx sensor is simplified. Such a NOx sensor isadvantageous from the viewpoint of cost and applicability to massproduction.

It is desirable that the first measuring electrode 28 and the secondmeasuring electrode 22 are arranged as nearly as possible in thediffusing direction. Accordingly, it is possible to decrease the changein electromotive force of the first electrochemical sensor cell, i.e.,the measuring error for the NOx concentration, caused by the change indistribution of oxygen concentration existing in the gas-diffusingdirection depending on the oxygen concentration in the measurement gas.

For example, porous Al₂ O₃ and ZrO₂ are used for the porous materiallayer 40 having the predetermined diffusion resistance, arranged tocover the first measuring electrode 28.

A second modified embodiment shown in FIG. 3 (A), (B) is different fromthe first modified embodiment shown in FIG. 2 (A) and FIG. 2 (B), whichis specifically characterized in that a first measuring electrode 28 iscovered with one porous material layer 40 having a predetermineddiffusion resistance for constructing the second diffusionrate-determining means and the second internal space, the firstmeasuring electrode 28 and a second measuring electrode 22 are arrangedin parallel with respect to an atmosphere-diffusing direction in aninternal space 42, and the first and second reference electrodes areprovided as one common reference electrode 50. Owing to the arrangementof the first measuring electrode 28 and the second measuring electrode22, it is possible to further decrease the change in electromotive forceof the first electrochemical sensor cell, i.e., the measuring error forthe NOx concentration, caused by the change in distribution of oxygenconcentration existing in the gas-diffusing direction depending on theoxygen concentration in the measurement gas. In the second modifiedembodiment, the first measuring electrode 28 and the second measuringelectrode 22 may be provided on the solid electrolyte layers 4a, 4crespectively to oppose to one another with the internal space 42interposed therebetween, while they are arranged in parallel withrespect to the atmosphere-diffusing direction in the internal space 42.

A third modified embodiment shown in FIG. 4 (A) is specificallycharacterized as follows, in addition to the feature of the firstembodiment shown in FIG. 1 (A), (B). Namely, a porous material layer 62having a predetermined diffusion resistance is provided around the firstmeasuring electrode 28 of the first electrochemical sensor cell,arranged in the second internal space 8. A third measuring electrode 60is provided in contact with the solid electrolyte layer 4a located onthe second internal space 8 so that the third measuring electrode 60 isopposed to the first measuring electrode 28 with the second internalspace 8 interposed therebetween. The third measuring electrode 60, thesolid electrolyte layers 4a, 4b, 4c, and the first reference electrode30 constitute a third electrochemical sensor cell for detecting, in thesecond internal space 8, the partial pressure of oxygen in theatmosphere introduced from the first internal space 6 into the secondinternal space 8 through the second diffusion rate-determining passage14. An electromotive force outputted from the third electrochemicalsensor cell is detected by a third potentiometer 64 to obtain a detectedvalue on the basis of which correction is performed for control of theelectrochemical pumping cell effected by the second electrochemicalsensor cell.

Namely, the partial pressure of oxygen in the measurement gas, which isintroduced from the first internal space 6 into the second internalspace 8 through the second diffusion rate-determining passage 14, isdetected by the third potentiometer 64 on the basis of the electromotiveforce of the third electrochemical sensor cell constructed by the thirdmeasuring electrode 60 arranged in the second internal space 8, thefirst reference electrode 30, and the solid electrolyte layers 4a, 4b,4c. The control of partial pressure of oxygen in the first internalspace 6, which is performed by the electrochemical pumping cellcontrolled by the second electrochemical sensor cell, is corrected onthe basis of a detected value of partial pressure of oxygen. Themeasurement gas introduced into the second internal space 8 diffusesunder the predetermined diffusion resistance of the porous materiallayer 62 arranged in the second internal space 8, and arrives at thefirst measuring electrode 28 arranged at the inner section of the porousmaterial layer 62. Thus NOx is reduced by the first measuring electrode28. The electromotive force, which is generated between the firstmeasuring electrode 28 and the first reference electrode 30, is measuredby the first potentiometer 32.

The control of the electrochemical pumping cell is corrected byregulating a target value of partial pressure of oxygen in theatmosphere in the first internal space 6 concerning the secondelectrochemical sensor cell which controls the electrochemical pumpingcell so that the electromotive force of the third electrochemical sensorcell is maintained to be a constant value. For example, when the partialpressure of oxygen in the atmosphere in the second internal space 8 israised, namely when the electromotive force of the third electrochemicalsensor cell is lowered, the target value of partial pressure of oxygenis lowered. Thus the electrochemical pumping cell is operated so thatthe partial pressure of oxygen in the atmosphere in the first internalspace 6 is lowered. Accordingly, the partial pressure of oxygen in theatmosphere in the first internal space 6, and consequently the partialpressure of oxygen in the atmosphere in the second internal space 8 arelowered.

More specifically, a system as shown in FIG. 4 (B) is employed. Atfirst, a voltage value of the third potentiometer 64 is compared with acomparison voltage corresponding to a target value of partial pressureof oxygen in the atmosphere in the second internal space 8, by using acomparator. A difference therebetween is amplified into a predeterminedmagnitude by using an amplifier, followed by being subjected to adelaying and waveform-converting process to avoid oscillation uponfeedback control so that a correcting comparison voltage is obtained.The correcting comparison voltage is applied in series to a comparisonvoltage for the second potentiometer 26 which corresponds to a targetvalue of partial pressure of oxygen in the atmosphere in the firstinternal space 6. Thus the comparison voltage for the secondpotentiometer 26 is corrected. After that, a voltage value of the secondpotentiometer 26 is compared with the corrected comparison voltage forthe second potentiometer 26 by using a comparator. A differencetherebetween is amplified by an amplifier into a predetermined magnitudeto obtain a voltage which is applied to the electrochemical pumping cellso that the electrochemical pumping cell is operated.

According to the NOx sensor as described above, even when the oxygenconcentration in the measurement gas is high, and hence the change inelectromotive force of the first electrochemical sensor cell, i.e., themeasuring error for the NOx concentration, which is caused by the changein distribution of oxygen concentration in the measurement gas dependingon the oxygen concentration in the measurement gas, is extremely large,the electrochemical pumping cell is controlled by the secondelectrochemical sensor cell on the basis of the value of partialpressure of oxygen in the atmosphere in the vicinity of the firstelectrochemical sensor cell. Accordingly, the value of partial pressureof oxygen is stably maintained to be constant. Therefore, it is possibleto further decrease the measuring error for the NOx concentrationresulting from the distribution of oxygen concentration in themeasurement gas. Moreover, the porous material layer 62 having thepredetermined diffusion resistance is provided around the firstmeasuring electrode 28 of the first electrochemical sensor cell.Accordingly, the sensitivity for detecting the NOx concentration is alsoimproved.

In the foregoing first embodiment, the concentration of NOx isdetermined by detecting the change in partial pressure of oxygen in thesecond internal space caused by oxygen produced by reducing ordecomposing NOx in the second internal space, as the change in value ofthe electromotive force outputted by the first electrochemical sensorcell. However, in the present invention, a NOx sensor and a method ofmeasuring NOx as described below are also advantageously adopted inorder to measure the NOx concentration in the measurement gas.

Namely, an electrochemical sensor cell is formed, which includes a firstmeasuring electrode arranged in a predetermined internal space, having aporous structure and functioning as a NOx-reducing catalyst as well. Aconstant current is allowed to flow so that oxygen is pumped out fromthe first measuring electrode to a first reference electrode, by using aconstant current power source provided on an electromotiveforce-outputting circuit comprising the first measuring electrode andthe first reference electrode of the electrochemical sensor cell. Underthis condition, NOx existing in an atmosphere in the internal space isreduced or decomposed in a porous structure of the first measuringelectrode. An electromotive force, which corresponds to a partialpressure of oxygen in an atmosphere in the porous structure, defined byoxygen produced by the reduction or decomposition of NOx, is detectedfrom an output from the electrochemical sensor cell to obtain a detectedvalue from which a NOx concentration in a measurement gas is determined.An example is shown in FIG. 5 (A), (B).

A second embodiment representatively exemplifies another NOx sensor(second NOx sensor) according to the present invention is shown in FIG.5 (A), (B). The second embodiment adopts a structure similar to thatshown in FIG. 2 (A) and FIG. 2 (B), which is specifically characterizedin that the function of the porous material layer 40 in FIG. 2 (A), (B)is possessed by a first measuring electrode 70, and a constant currentpower source is provided on an electromotive force-outputting circuitcomprising the first measuring electrode and the first referenceelectrode of the electrochemical sensor cell in FIG. 2 (A), (B) to allowa constant current to flow so that oxygen is pumped out from the firstmeasuring electrode to the first reference electrode, and thus a partialpressure of oxygen in an atmosphere in a porous structure of the firstmeasuring electrode comes to a predetermined value at which NOx isreduced.

Namely, in the second embodiment, the first measuring electrode 70having the porous structure also functions as both of the seconddiffusion rate-determining means and the second internal space. Atfirst, the atmosphere of the measurement gas with its partial pressureof oxygen controlled in an internal space 42 is introduced into theinside of the first measuring electrode 70 having the porous structureunder a predetermined diffusion resistance.

After that, the oxygen in the atmosphere of the measurement gasintroduced into the inside of the first measuring electrode 70 havingthe porous structure is pumped out from the inside of the firstmeasuring electrode 70 to a reference gas-existing space 10 by the aidof the first reference electrode 30, in a flow amount alwayscorresponding to a previously set current value of the constant currentpower source, owing to the pumping action effected by the constantcurrent power source 72 provided on the electromotive force-outputtingcircuit comprising the first measuring electrode 70 and the firstreference electrode 30 of the first electrochemical sensor cell.However, the first measuring electrode 70 has the porous structurehaving the diffusion resistance. Accordingly, pressure loss occurs inthe flow of oxygen caused by diffusion effected by the pumping action ofthe constant current power source 72. The partial pressure of oxygen inthe porous structure is lower than that in the internal space 42 by anamount corresponding to the pressure loss.

NOx in the atmosphere diffuses into the first measuring electrode 70under the predetermined diffusion resistance. NOx is reduced ordecomposed, in the vicinity of the surface of the first measuringelectrode 70, by the first measuring electrode 70 having the porousstructure which also functions as a NOx-reducing catalyst. The partialpressure of oxygen in the atmosphere in the porous structure of thefirst measuring electrode 70 is increased by oxygen produced by thereduction or decomposition of NOx. When the NOx concentration is zero,the partial pressure of oxygen in the porous structure is lower thanthat in the internal space 42 by the amount corresponding to thepressure loss. Since the first measuring electrode 70 has the diffusionresistance, the produced oxygen in the first measuring electrodescarcely diffuses into the internal space 42 when the amount of increasein partial pressure of oxygen is smaller than the amount correspondingto the pressure loss as a matter of course, and even when the former islarger than the latter. Therefore, the change in NOx concentration inthe measurement gas results in a large change in partial pressure ofoxygen in the porous structure.

The constant current power source 72 greatly changes the voltage to beapplied to the first electrochemical sensor cell, in order to allow thecurrent of the previously set value to continuously flow through theelectromotive force-outputting circuit in response to the change inpartial pressure of oxygen in the porous structure of the firstmeasuring electrode 70. The change in voltage, which is applied to thefirst electrochemical sensor cell by the constant current power source72 corresponding to the partial pressure of oxygen in the atmosphere inthe porous structure, defined by the produced oxygen, is detected by afirst potentiometer 32 provided on the electromotive force-outputtingcircuit of the first electrochemical sensor cell. Accordingly, even aslight amount of produced oxygen can be measured as a large change involtage. Moreover, the diffusion of produced oxygen into the internalspace is suppressed to be less than that of the NOx sensor concerningthe first embodiment. Thus the sensitivity for detecting the NOxconcentration is more increased than that of the NOx sensor concerningthe first embodiment.

Desirably, the partial pressure of oxygen in the internal space 42 isset to be not more than about 1/100 of the NOx concentration in themeasurement gas in order to increase the sensitivity for detecting theNOx concentration. If NOx is reduced in the internal space 42 in thisprocedure, an alloy cermet electrode composed of, for example, Pt/Au isused as the second measuring electrode 22 and the inner pumpingelectrode 16 in order to lower the ability to reduce NOx.

In the NOx sensor having the structure as described above, the firstmeasuring electrode 70 having the porous structure also functions as thesecond diffusion rate-determining means and the second internal space.Therefore, it is unnecessary to separately provide a diffusionrate-determining means. Thus the structure of the NOx sensor issimplified, providing an advantage from the viewpoint of cost andapplicability to mass production. Moreover, when a diffusionrate-determining means, which is, for example, a diffusionrate-determining layer composed of a porous material layer, isseparately provided on the first measuring electrode 70, it is possibleto more effectively suppress the diffusion of oxygen produced by thereduction or decomposition of NOx, from the inside of the firstmeasuring electrode 70 to the internal space 42. Thus the sensitivityfor detecting the NOx concentration is further improved.

As for the NOx sensor according to the second embodiment, in order todecrease the change in electromotive force of the first electrochemicalsensor cell, i.e., the measuring error for the NOx concentration, causedby the change in distribution of oxygen concentration existing in thegas-diffusing direction depending on the oxygen concentration in themeasurement gas, it is desirable that the first measuring electrode 70and the second measuring electrode 22 are arranged in parallel withrespect to the direction of diffusion of the atmosphere in the internalspace 42, in the same manner as the second embodiment of the firstembodiment.

A modified embodiment of the second embodiment, shown in FIG. 6 isdifferent from the second embodiment shown in FIG. 5 (A) and FIG. 5 (B),which is characterized as follows. Namely, the internal space 42 in FIG.5 (A), (B) is divided into a first internal space 86 communicating withthe external measurement gas-existing space through a first diffusionrate-determining passage 12, and a second internal space 88 into whichan atmosphere in the first internal space 86 is introduced through asecond diffusion rate-determining passage 84 under a predetermineddiffusion resistance. The electrochemical pumping cell is located on thefirst internal space 86, while the first electrochemical sensor cell islocated on the second internal space 88. An inner subsidiary pumpingelectrode 82 is provided in contact with the solid electrolyte layer 4alocated on the second internal space 88. The inner subsidiary pumpingelectrode 82, the solid electrolyte layers 4a, 4b, 4c, and the firstreference electrode 30 constitute a subsidiary oxygen-pumping cell forlowering a partial pressure of oxygen in the atmosphere introduced fromthe first internal space 86 into the second internal space 88 to adegree sufficient to reduce NOx in a porous structure of a firstmeasuring electrode 80 of the first electrochemical sensor cell. Thesubsidiary oxygen-pumping cell is operated by an external DC powersource 90.

Namely, the measurement gas introduced from the first internal space 86through the second diffusion rate-determining passage 84 undergoes thepumping action effected by the subsidiary oxygen-pumping cellconstructed by the inner subsidiary pumping electrode 82 arranged in thesecond internal space 88, the first reference electrode 30, and thesolid electrolyte layers 4a, 4b, 4c. Thus the second internal space 88is controlled to have a low and constant value of partial pressure ofoxygen. The low and constant value of partial pressure of oxygen isstably maintained to be constant even when the oxygen concentration inthe measurement gas is high, and hence the change in electromotive forceof the first electrochemical sensor cell, i.e., the measuring error forthe NOx concentration, caused by the change in distribution of oxygenconcentration in the measurement gas depending on the oxygenconcentration in the measurement gas, is extremely large. Accordingly,it is possible to further decrease the measuring error for the NOxconcentration, caused by the distribution of oxygen concentration in themeasurement gas. Moreover, the NOx concentration in the measurement gascan be accurately measured in a state in which the partial pressure ofoxygen in the first internal space 86 is previously set to be apredetermined value which is sufficiently higher than that in the secondinternal space 88. Accordingly, even when inflammable gases such as CO,HC, and H₂ are present in the measurement gas in a mixed manner, theinflammable gases can be removed by oxidation in the first internalspace 86. Thus it is possible to decrease the measuring error for theNOx concentration, due to interference by the inflammable gases in thesecond internal space 88, as small as possible. It is needless to saythat the subsidiary oxygen-pumping cell can be equivalently applied andinstalled for the NOx sensor (first NOx sensor) according to the presentinvention, and thus an equivalent effect can be obtained.

The NOx sensors of the first and second embodiments shown in FIG. 1 (A),(B) and FIG. 5 (A), (B) respectively were used under the followingcondition. Namely, the pumping voltage of the electrochemical pumpingcell was 430 mV. The partial pressure of oxygen in the atmospheres inthe first internal space 6 or the internal space 42 was controlled to be10⁻¹⁰ atm. In this state, NO as a NOx component in a measurement gascomprising 5% O₂ in a carrier gas of N₂ was changed in a range of 0.8 to40 ppm. The change in electromotive force between the first measuringelectrode 28 or 70 and the reference electrode 30, obtained under thiscondition, is shown in FIG. 7. As clarified from the result shown inFIG. 7, the electromotive force changes in a range of about 15 mV in thefirst embodiment, or in a range of about 30 mV in the second embodiment,in a range of NO concentration of 1 to 10 ppm. Accordingly, a largechange in signal can be detected by the first potentiometer 32 even whenthe NO concentration in the measurement gas is low.

When the NO concentration is determined by measuring the electromotiveforce as performed by the NOx sensor according to the present invention,it is possible to select the sensitivity to NO and the measuring rangeby controlling the diffusion resistance concerning the first measuringelectrode 28. As for the result shown in FIG. 7, the first embodimentconcerns a case of the use of a sum of diffusion resistances of thediffusion rate-determining passage 14 having a cross-sectional area of0.2 mm² and the measuring electrode 28 itself comprising a porous cermethaving an electrode thickness of 10 μm and a porosity of 40%. The secondembodiment concerns a case of the use of only a diffusion resistance ofthe measuring electrode 70 itself comprising a porous cermet having anelectrode thickness of 10 μm and a porosity of 40%. The sensitivity toNO is increased by increasing the diffusion resistance by providing, forexample, a porous diffusion rate-determining section such as a porousalumina layer and a porous zirconia layer stacked on the measuringelectrode 28, 70. However, it is difficult to perform measurement in ahigh concentration region. O₂ produced by reduction on the electrode isapt to remain in the electrode area, and the oxygen concentration is aptto increase. Therefore, the change in electromotive force becomes large.However, the oxygen concentration in the electrode area is apt toincrease due to O₂ produced by reduction. For this reason, reductiondoes not occur if the oxygen concentration exceeds a certain level.Accordingly, it is difficult to perform measurement in a highconcentration region. Therefore, the sensitivity and the measuring rangeshould be determined by appropriately setting the diffusion resistancedepending on the feature of a region in which the NO concentration ismeasured.

It is needless to say that the present invention may be carried out invarious altered, corrected, and improved forms on the basis of theknowledge of those skilled in the art. It should be understood that anyof such forms belongs to the category of the present invention, within arange without deviating from the spirit of the present invention.

As clarified from the foregoing explanation, according to the NOx sensorand the method of measuring NOx concerning the present invention, evenwhen the NOx concentration in a measurement gas is low in a degree ofseveral ppm, an electromotive force is detected, which corresponds to apartial pressure of oxygen in an atmosphere, defined by oxygen producedby reducing or decomposing NOx, in the presence of the diffusionrate-determining means having a predetermined diffusion resistance. Thusa large change in electromotive force, i.e., a large change in signal isobtained as exemplified in FIG. 7 even when the measurement gascomponent has a low concentration. Moreover, measurement can beperformed continuously and accurately with good response over a longperiod of time.

What is claimed is:
 1. A NO sensor for measuring an amount of NOx in ameasurement gas by measuring an amount of oxygen produced by reducingNOx in said measurement gas with a NOx-reducing catalyst, said NOxsensor comprising:a first internal space communicating with an externalmeasurement gas-existing space; a first diffusion rate-determining meansfor introducing said measurement gas from said measurement gas-existingspace into said first internal space under a predetermined diffusionresistance; an electrochemical pumping cell comprising a first oxygenion-conductive solid electrolyte and a pair of electrodes provided incontact therewith, for pumping out oxygen from said first internal spaceby applying electric power between said pair of electrodes so that apartial pressure of oxygen in an atmosphere in said first internal spaceis controlled to have a predetermined low value at which NOx is notsubstantially reduced; a second internal space communicating with saidfirst internal space and comprising said NOx-reducing catalyst arrangedtherein; a second diffusion rate-determining means for introducing saidcontrolled atmosphere in said first internal space into said secondinternal space under a predetermined diffusion resistance; anelectrochemical sensor cell comprising a second oxygen ion-conductivesolid electrolyte and a pair of electrodes provided in contacttherewith, for reducing NOx existing in an atmosphere in said secondinternal space with said NOx-reducing catalyst, and outputting anelectromotive force corresponding to a partial pressure of oxygen insaid atmosphere in said second internal space, defined by oxygenproduced by said reduction of NOx; and a voltage-detecting means fordetecting said electromotive force outputted from said electrochemicalsensor cell.
 2. The NOx sensor according to claim 1, wherein saidNOx-reducing catalyst also serves as said electrode arranged in saidsecond internal space, of said pair of electrodes for constituting saidelectrochemical sensor cell.
 3. The NOx sensor according to claim 1,comprising a sensor element having an integrated structure includingsaid first and second oxygen ion-conductive solid electrolytes, whereinsaid sensor element integrally includes said first and second internalspaces, said first and second diffusion rate-determining means, saidelectrochemical pumping cell, and said electrochemical sensor cell. 4.The NOx sensor according to claim 1, comprising a sensor element havingan integrated structure including said first and second oxygenion-conductive solid electrolytes, and a partial oxygenpressure-detecting means for detecting said partial pressure of oxygenin said atmosphere in said first internal space, wherein said partialpressure of oxygen in said atmosphere in said first internal space iscontrolled by controlling an amount of electric power applicationbetween said pair of electrodes of said electrochemical pumping cell onthe basis of a value of partial pressure of oxygen detected by saidpartial oxygen pressure-detecting means.
 5. The NOx sensor according toclaim 4, wherein a reference gas-existing space is provided in saidintegrated structure of said sensor element independently from saidfirst and second internal spaces, and said partial oxygenpressure-detecting means is constructed by a second electrochemicalsensor cell comprising an oxygen ion-conductive solid electrolyteextending between said reference gas-existing space and said firstinternal space, a second reference electrode provided in contact withsaid solid electrolyte located on said reference gas-existing space, anda second measuring electrode provided in contact with said solidelectrolyte located on said first internal space.
 6. The NOx sensoraccording to claim 5, wherein said reference gas-existing space is openat an aperture of said sensor element exposed to atmospheric air, andatmospheric air as a reference gas is introduced into said referencegas-existing space through said aperture.
 7. The NOx sensor according toclaim 5, wherein said electrochemical sensor cell for outputting saidelectromotive force corresponding to said partial pressure of oxygen insaid atmosphere in said second internal space uses said oxygenion-conductive solid electrolyte extending between said second internalspace and said reference gas-existing space, as said second oxygenion-conductive solid electrolyte, and said pair of electrodes of saidelectrochemical sensor cell are constructed by a first measuringelectrode provided in contact with said solid electrolyte located onsaid second internal space and a first reference electrode provided incontact with said solid electrolyte located on said referencegas-existing space.
 8. The NOx sensor according to claim 7, wherein alayer of said NOx-reducing catalyst is provided on said first measuringelectrode.
 9. The NOx sensor according to claim 1, wherein said seconddiffusion rate-determining means is composed of a porous material havinga predetermined diffusion resistance.
 10. The NOx sensor according toclaim 1, wherein said second internal space is composed of a porousmaterial having a predetermined diffusion resistance.
 11. The NOx sensoraccording to claim 7, wherein said second diffusion rate-determiningmeans and said second internal space are constructed by one porousmaterial having a predetermined diffusion resistance, and arranged insaid first internal space, and said first measuring electrode and saidsecond measuring electrode are arranged adjacent to one another withrespect to a direction of diffusion of said atmosphere in said firstinternal space.
 12. The NOx sensor according to claim 11, wherein saidfirst measuring electrode and said second measuring electrode arearranged in parallel with respect to said direction of diffusion of saidatmosphere in said first internal space.
 13. The NOx sensor according toclaim 5, further comprising an electrochemical sensor cell fordetecting, in said second internal space, said partial pressure ofoxygen in said atmosphere introduced from said first internal space intosaid second internal space through said second diffusionrate-determining means, wherein control of said electrochemical pumpingcell, which is effected by said electrochemical sensor cell forconstituting said partial oxygen pressure-detecting means, is correctedon the basis of a value of partial pressure of oxygen detected by saidelectrochemical sensor cell.
 14. The NOx sensor according to claim 13,wherein a porous material layer having a predetermined diffusionresistance is provided around said first measuring electrode of saidelectrochemical sensor cell arranged in said second internal space, foroutputting said electromotive force corresponding to said partialpressure of oxygen in said atmosphere in said second internal space. 15.The NOx sensor according to claim 1, further comprising a heating meanscapable of heating said first internal space and said second internalspace to a predetermined temperature respectively.
 16. A NO sensor,based on the use of a sensor element having an integrated structurecomprising oxygen ion-conductive solid electrolytes, for measuring anamount of NOx in a measurement gas by measuring an amount of oxygenproduced by reducing NOx in said measurement gas with a NOx-reducingcatalyst arranged in an internal space provided in said sensor element,said NOx sensor comprising:a diffusion rate-determining means forintroducing said measurement gas from an external measurementgas-existing space into said internal space in said sensor element undera predetermined diffusion resistance; an electrochemical pumping cellcomprising an oxygen ion-conductive solid electrolyte of said sensorelement and a pair of electrodes provided in contact therewith, forpumping out oxygen from said internal space by applying electric powerbetween said pair of electrodes so that a partial pressure of oxygen inan atmosphere in said internal space is controlled to have apredetermined low value at which NOx is not substantially reduced; anelectrochemical sensor cell comprising an oxygen ion-conductive solidelectrolyte of said sensor element, and a measuring electrode and areference electrode provided in contact therewith, said measuringelectrode being located on said internal space and having a porousstructure to function as said NOx-reducing catalyst as well, and saidreference electrode being located on a reference gas-existing spaceprovided in said sensor element, wherein NOx existing in an atmospherein said internal space is reduced in said porous structure of saidmeasuring electrode, and an electromotive force is outputtedcorresponding to a partial pressure of oxygen in an atmosphere in saidporous structure, defined by oxygen produced by said reduction of NOx; aconstant current power source, provided on an electromotiveforce-outputting circuit including said measuring electrode and saidreference electrode of said electrochemical sensor cell, for allowing aconstant current to flow so that oxygen is pumped out from saidmeasuring electrode to said reference electrode, and allowing saidpartial pressure of oxygen in said atmosphere in said porous structureof said measuring electrode to have a predetermined value at which NOxis reduced; and a voltage-detecting means, provided on saidelectromotive force-outputting circuit of said electrochemical sensorcell, for detecting said electromotive force outputted from saidelectrochemical sensor cell.
 17. The NOx sensor according to claim 16,further comprising a partial oxygen pressure-detecting means fordetecting said partial pressure of oxygen in said atmosphere in saidinternal space, wherein said partial pressure of oxygen in saidatmosphere in said internal space is controlled by controlling an amountof electric power application between said pair of electrodes of saidelectrochemical pumping cell on the basis of a value of partial pressureof oxygen detected by said partial oxygen pressure-detecting means. 18.The NOx sensor according to claim 16, wherein said partial oxygenpressure-detecting means is constructed by said oxygen ion-conductivesolid electrolyte of said sensor element, a measuring electrode locatedon said internal space, provided in contact therewith, and a referenceelectrode located on said reference gas-existing space.
 19. The NOxsensor according to claim 16, wherein a porous material layer having apredetermined diffusion resistance is provided around said measuringelectrode of said electrochemical sensor cell.
 20. The NOx sensoraccording to claim 18, wherein said measuring electrode of saidelectrochemical sensor cell and said measuring electrode of said partialoxygen pressure-detecting means are arranged adjacent to one anotherwith respect to a direction of diffusion of said atmosphere in saidinternal space.
 21. The NOx sensor according to claim 18, wherein saidmeasuring electrode of said electrochemical sensor cell and saidmeasuring electrode of said partial oxygen pressure-detecting means arearranged in parallel with respect to a direction of diffusion of saidatmosphere in said internal space.
 22. The NOx sensor according to claim16, wherein said internal space is divided into a first internal spacecommunicating with said external measurement gas-existing space throughsaid first diffusion rate-determining means, and a second internal spaceinto which an atmosphere in said first internal space is introducedthrough a second diffusion rate-determining means under a predetermineddiffusion resistance, said electrochemical pumping cell is located onsaid first internal space, said electrochemical sensor cell is locatedon said second internal space, and said NOx sensor further comprises asubsidiary oxygen-pumping means for lowering a partial pressure ofoxygen in said atmosphere introduced from said first internal space intosaid second internal space, to a degree sufficient to reduce NOx in saidporous structure of said measuring electrode of said electrochemicalsensor cell.
 23. The NOx sensor according to claim 16, wherein saidinternal space is divided into a first internal space communicating withsaid external measurement gas-existing space through said firstdiffusion rate-determining means, and a second internal space into whichan atmosphere in said first internal space is introduced through asecond diffusion rate-determining means under a predetermined diffusionresistance, said electrochemical pumping cell is located on said firstinternal space, said electrochemical sensor cell is located on saidsecond internal space, said NOx sensor further comprises a subsidiaryoxygen-pumping means for lowering a partial pressure of oxygen in saidatmosphere introduced from said first internal space into said secondinternal space, to a degree sufficient to reduce NOx in said porousstructure of said measuring electrode of said electrochemical sensorcell, and said subsidiary oxygen-pumping means is constructed by saidoxygen ion-conductive solid electrolytes of said sensor element and apair of electrodes provided in contact therewith.
 24. The NOx sensoraccording to claim 16, further comprising a heating means capable ofheating said internal spaces to a predetermined temperature.
 25. Amethod of measuring NOx, comprising the steps of:introducing ameasurement gas from an external measurement gas-existing space into afirst internal space under a predetermined diffusion resistance;controlling a partial pressure of oxygen in an atmosphere in said firstinternal space to have a predetermined low value at which NO is notsubstantially reduced, by the aid of an oxygen-pumping action effectedon said first internal space by an electrochemical pumping cell;introducing said controlled atmosphere in said first internal space intoa second internal space under a predetermined diffusion resistance;reducing, in said second internal space, NOx existing in an atmospherewith a NOx-reducing catalyst; and outputting, with the use of anelectrochemical sensor cell, an electromotive force corresponding to apartial pressure of oxygen in said atmosphere in said second internalspace, defined by oxygen produced by the reducing step to obtain adetected output value from which an amount of NOx in said measurementgas is determined.
 26. The method according to claim 25, wherein saidpartial pressure of oxygen in said atmosphere in said first internalspace is controlled to have said constant value by detecting saidpartial pressure of oxygen in said atmosphere in said first internalspace to obtain a detected value on the basis of which a power sourcevoltage is changed so that said oxygen-pumping action effected by saidelectrochemical pumping cell is controlled.
 27. A method of measuringNOx, comprising the steps of:introducing a measurement gas from anexternal measurement gas-existing space into an internal space under apredetermined diffusion resistance; controlling a partial pressure ofoxygen in an atmosphere in said internal space to have a predeterminedlow value at which NO is not substantially reduced, by the aid of anoxygen-pumping action effected on said internal space by anelectrochemical pumping cell; reducing NOx in a porous structure byallowing a constant current to flow through an electrochemical sensorcell comprising a measuring electrode having said porous structurelocated on said internal space so that a partial pressure of oxygen inan atmosphere in said porous structure has a predetermined value atwhich NOx is reduced; and outputting, with the use of saidelectrochemical sensor cell, an electromotive force corresponding to apartial pressure of oxygen defined by oxygen produced by the reducingstep to obtain a detected output value from which an amount of NOx insaid measurement gas is determined.
 28. The method according to claim27, wherein said partial pressure of oxygen in said atmosphere in saidinternal space is controlled to have said constant value by detectingsaid partial pressure of oxygen in said atmosphere in said internalspace to obtain a detected value on the basis of which a power sourcevoltage is changed so that said oxygen-pumping action effected by saidelectrochemical pumping cell is controlled.